Why Can’t Rodents Vomit? A Comparative Behavioral, Anatomical, and Physiological Study

Abstract

The vomiting (emetic) reflex is documented in numerous mammalian species, including primates and carnivores, yet laboratory rats and mice appear to lack this response. It is unclear whether these rodents do not vomit because of anatomical constraints (e.g., a relatively long abdominal esophagus) or lack of key neural circuits. Moreover, it is unknown whether laboratory rodents are representative of Rodentia with regards to this reflex. Here we conducted behavioral testing of members of all three major groups of Rodentia; mouse-related (rat, mouse, vole, beaver), Ctenohystrica (guinea pig, nutria), and squirrel-related (mountain beaver) species. Prototypical emetic agents, apomorphine (sc), veratrine (sc), and copper sulfate (ig), failed to produce either retching or vomiting in these species (although other behavioral effects, e.g., locomotion, were noted). These rodents also had anatomical constraints, which could limit the efficiency of vomiting should it be attempted, including reduced muscularity of the diaphragm and stomach geometry that is not well structured for moving contents towards the esophagus compared to species that can vomit (cat, ferret, and musk shrew). Lastly, an in situ brainstem preparation was used to make sensitive measures of mouth, esophagus, and shoulder muscular movements, and phrenic nerve activity–key features of emetic episodes. Laboratory mice and rats failed to display any of the common coordinated actions of these indices after typical emetic stimulation (resiniferatoxin and vagal afferent stimulation) compared to musk shrews. Overall the results suggest that the inability to vomit is a general property of Rodentia and that an absent brainstem neurological component is the most likely cause. The implications of these findings for the utility of rodents as models in the area of emesis research are discussed.

Autonomic changes induced by provocative motion in rats bred for high (HAB) and low (LAB) anxiety-related behavior: Paradoxical responses in LAB animals

“…However, it should be noted that the relationship between anxiety and nausea is based primarily on the higher probability of occurrence of nausea in subjects with higher anxiety levels [2,21,23,37,42], and correlations between degree of anxiety and magnitude of nausea are weak [27,44]; the present A C C E P T E D M A N U S C R I P T study only models the latter situation. Taking the results at face value, motion clearly produced hypothermia in all three lines, and this is consistent with previous studies of hypothermia induced by a number of emetic stimuli in both rats (lacking an emetic reflex [19]) and Suncus murinus (with an emetic reflex) [9,17,35]. (and bradycardia) response in LAB compared to HAB and NAB is due to an increased sensitivity in the vestibular-thermoregulatory/cardiovascular pathways that are psychogenetically selected by the elevated plus-maze test.…”

“…We have recently demonstrated that similar hypothermic responses are present in rats (lacking an emetic reflex [19]) and in house musk shrews (Suncus murinus, an insectivore possessing emetic reflex) [9,35]. We have proposed, based on several arguments, that the drop in body temperature might represent a biomarker of a nausea-like state in laboratory animals.…”

Infantile hypertrophic pyloric stenosis (IHPS): A study of its pathophysiology utilizing the newborn hph-1 mouse model of the disease

“…However, rodents lack the vomiting reflex, and this precludes them from getting as malnourished and dehydrated as seen in human infants with this condition (12,43).…”

Fasting does not induce gastric emptying in rats

“…Some reasons for the inability to vomit are: 1) diaphragmatic structure relatively less muscular 3,12 ; 2) esophagus length relatively longer than non-emetic species 13 ; 3) stomach geometry with absence of a funnel shape 3 ; 4) absent brainstem neurological component of vomit 3,14 .…”

“…However, there is evidence that small murines, such as rats and mice, do not exhibit the ability to vomit, and in case of fasting they have an eating disorder that involves the appetite perversion, demonstrated by eating non-food substances, such as soil and their bedding 2,3 . Thus, food deprivation becomes just a stress for the animal trial, and can influence in some experiments because of the changes in carbohydrate metabolism 4 .…”